10) Give Side Characters A Purpose
Didn’t it seem like Lois Lane’s character arc took a step backwards in JL? Her previous appearances in Man of Steel and Batman V Superman included purposeful storylines in which she brought Superman out of hiding, gave Clark the motivation to embrace his superhuman gifts, and uncovered the major conspiracy at play by Lex Luthor. In Justice League, though, she talks with Martha Kent about Clark, calms Clark down, and then tells him to go save the League. She, along with most of the other side players, have very little to do and in turn, feel pretty much wasted.
Instead of feeling like her own character, she feels more like a means to get Superman through his own arc. Same goes for Henry Allen with Flash and Mera with Aquaman. Alfred fits a similar role, but he at least has enough screentime to be his own person beyond his “helping Bruce grow” scenes. It makes sense why these characters wouldn’t get major focus (as they’re likely being saved for each member’s standalone), but their lack of distinct purpose is difficult to ignore.
The focus of the film may be on the League members, but that doesn’t mean these side characters should be used simply as plot devices or stepping stones.